1. Field of the Invention
This invention relates to communication systems, and particularly to systems and methods for generating and transmitting data signals to the surface of the earth while drilling a borehole, wherein the transmitted signal is maximized and the probability of the system being jammed by drilling fluid particulates is minimized.
2. Description of the Related Art
It is desirable to measure or "log", as a function of depth, various properties of earth formations penetrated by a borehole while the borehole is being drilled, rather than after completion of the drilling operation. It is also desirable to measure various drilling and borehole parameters while the borehole is being drilled. These technologies are known as logging-while-drilling and measurement-while-drilling, respectively, and will hereafter be referred to collectively as "MWD". Measurements are generally taken with a variety of sensors mounted within a drill collar above, but preferably close, to a drill bit which terminates a drill string. Sensor responses, which are indicative of the formation properties of interest or borehole conditions or drilling parameters, are then transmitted to the surface of the earth for recording and analysis.
Various systems have been used in the prior art to transmit sensor response data from downhole drill string instrumentation to the surface while drilling a borehole. These systems include the use of electrical conductors extending through the drill string, and acoustic signals that are transmitted through the drill string. The former technique requires expensive and often unreliable electrical connections that must be made at every pipe joint connection in the drill string. The latter technique is rendered ineffective under most conditions by "noise" generated by the actual drilling operation.
The most common technique used for transmitting MWD data utilizes drilling fluid as a transmission medium for acoustic waves modulated downhole to represent sensor response data. The modulated acoustic waves are subsequently sensed and decoded at the surface of the earth. The drilling fluid or "mud" is typically pumped downward through the drill string, exits at the drill bit, and returns to the surface through the drill string-borehole annulus. The drilling fluid cools and lubricates the drill bit, provides a medium for removing drill bit cuttings to the surface, and provides a hydrostatic pressure head to balance fluid pressures within formations penetrated by the drill bit.
Drilling fluid data transmission systems are typically classified as one of two species depending upon the type of pressure pulse generator used, although "hybrid" systems have been disclosed. The first species uses a valving system to generate a series of either positive or negative, and essentially discrete, pressure pulses which are digital representations of transmitted data. The second species, an example of which is disclosed in U.S. Pat. No. 3,309,656, comprises a rotary valve or "mud siren" pressure pulse generator which repeatedly interrupts the flow of the drilling fluid, and thus causes varying pressure waves to be generated in the drilling fluid at a carrier frequency that is proportional to the rate of interruption. Downhole sensor response data is transmitted to the surface of the earth by modulating the acoustic carrier frequency.
U.S. Pat. No. 5,182,730 discloses a first species of data transmission system which uses the bits of a digital signal from a downhole sensor to control the opening and closing of a restrictive valve in the path of the mud flow. Such a transmission may reduce interference from drilling fluid circulation pump or pumps, and interference from other drilling related noises. The data transmission rate of such a system is, however, relatively slow as is well known in the art.
U.S. Pat. No. 4,847,815, which is incorporated herein by reference, discloses an additional example of the second species of data transmission system comprising a downhole rotary valve or mud siren. The data transmission rate of this system is relatively high, but it is susceptible to extraneous noise such as noise from the drilling fluid circulation pump. Additionally, for low flows, deep wells, small diameter drill strings, and/or high viscosity muds, this system requires small gap settings for maximizing signal pressure at the modulator. Under these conditions the system is susceptible to plugging or "jamming" by solid particulate material in the drilling mud, such as lost circulation material "LCM", which will be subsequently defined.
U.S. Pat. No. 5,375,098, also incorporated herein by reference, discloses an improved rotary valve system which includes apparatus and methods for suppressing noise. Although data transmission rates are relatively high and relatively free of noise distortion, this rotary valve system is still susceptible to jamming by solid particulates at small gap settings.
The effects of the above parameters are shown by the signal strength relationship from Lamb, H., Hydrodynamics, Dover, New York, N.Y. (1945), pages 652-653, which is:
S=S.sub.o exp[-4.pi.F(D/d).sup.2 (.mu./K)]
where
S=signal strength at a surface transducer; PA1 S.sub.o =signal strength at the downhole modulator; PA1 F=carrier frequency of the MWD signal expressed in Hertz; PA1 D=measured depth between the surface transducer and the downhole modulator; PA1 d=inside diameter of the drill pipe (same units as measured depth); PA1 .mu.=plastic viscosity of the drilling fluid; and PA1 K=bulk modulus of the volume of mud above the modulator, PA1 S.sub.o =signal strength at the downhole modulator; PA1 .rho..sub.mud =density of the drilling fluid; PA1 Q=volume flow rate of the drilling fluid; and PA1 A=the flow area with the modulator in the "closed" position, a function of the gap setting.
and by the modulator signal pressure relationship EQU S.sub.o.varies.(.rho..sub.mud.times.Q.sup.2)/A.sup.2
where
U.S. Pat. No. 5,583,827 discloses a rotary valve telemetry system which generates a carrier signal of constant frequency, and sensor data are transmitted to the surface by modulating the amplitude rather than the frequency of the carrier signal. Amplitude modulation is accomplished by varying the spacing or "gap" between a rotor and stator component of the valve. Gap variation is accomplished by a system which induces relative axial movement between rotor and stator depending upon the digitized output of a downhole sensor. The '827 patent also discloses the use of a plurality of such valve systems operated in tandem. The system is, however, mechanically and operationally complex, and is also subject to the same jamming limitations as previously discussed when operating at the small gap positions necessary for generating maximum signal amplitude.
All drill string components, including MWD tools, should be designed to allow the continuous flow of solids and additives suspended in the drilling fluid. As discussed previously, an important example of an additive is lost circulation material or "LCM". One type of common LCM is "medium nut plug" which is a material used to control lost circulation of drilling fluids into certain types of formations penetrated by the drill bit during the drilling operation. This material can be of vital importance in drilling a well when it is used to plug fractures in formations, to isolate incompetent formations to which drilling fluid can be lost, or when drilling parameters result in too much overbalance pressure in the wellbore annulus with respect to the formation pressure. If loss of the drilling fluid occurs, the hydrostatic balance of the well may be disrupted and containment of the subsurface formation pressure may be lost. This has extreme negative safety implications for a rig and crew since loss of well control can lead to a "kick" and possibly a "blow-out" of the well. In view of these drilling mechanics and safety aspects, LCM such as medium nut plug is required in some drilling operations. Drilling equipment, including MWD equipment, must be able to pass LCM. As a result, the passage of medium nut plug is also a commonly accepted standard by which particulate performance of MWD tools is measured.
If jamming and plugging of the drill string occurs during flow of LCM in controlling lost circulation, the drill string must be removed from the well. This is a costly and complex operation, especially if the well and the downhole pressures are not stable. It is vital, therefore, to maintain the ability to transport LCM downhole via the drill string to arrest lost circulation problems in the well. LCM must, therefore, pass through all elements of the drill string, including the pressure pulse generator of a MWD tool.
Prior art rotary valve type pressure pulse modulators have used a lateral gap between the stator and rotor of the modulator to provide a flow area for drilling fluid, even when the modulator is in the "closed" position. As a result, the modulator is never completely closed as the drilling fluid must maintain a continuous flow for satisfactory drilling operations to be conducted. Thus, drilling fluid and particulate additives or debris must pass through the lateral gap of the modulator when it is in the closed position. In the prior art designs, the lateral gap has been limited to certain minimum values. At lateral gap settings below the minimum value, performance of the data telemetry system is degraded with respect to LCM tolerance such that jamming or plugging of the drill string may occur. Conversely, it is required that the lateral gap and associated closed flow area be as small as practical in order to maximize telemetry signal strength, which is proportional to the difference in differential pressure across the modulator when the modulator in the fully "open" and fully "closed" positions. Signal strength must be maximized at the MWD tool in order to maintain signal strength at the surface when low drilling fluid flow rates, increased well depths, smaller drill string cross sections, and/or high mud viscosity are mandated by the geological objective and particular drilling environment encountered. If the gap is reduced to less than the size of any particulate additives, there is increased difficulty in transporting these additives or debris through the modulator. At a certain point, depending upon the setting of the lateral gap between the rotor and the stator, the particle size and concentration, particle accumulation, packing and plugging of the drill string can occur. Additionally, at lower modulator frequencies, the amount of accumulation will be greater since the modulator is in the "closed" position for a longer period of time. Differential pressure will force the particles into the gap where they may wedge and jam the modulator. When this happens, the modulator rotor may malfunction, jam in the closed position, and the drill string may be packed off and plugged upstream from the modulator.